The present invention relates generally to hydraulic circuits for hydraulic motors and hydraulic brakes and to methods of controlling hydraulic motors and hydraulic brakes using a hydraulic circuit. More particularly, the present invention relates to a hydraulic circuit and method used to activate and deactivate hydraulic motors and hydraulic brakes, including hydraulic motors and brakes used on load carrying vehicles that must operate with precision on inclined surfaces.
Hydraulic motors and brakes are frequently used in industrial vehicles, including forklifts, scissor lifts, and mobile platforms. Although such industrial vehicles generally operate at low speeds, safety and other functional requirements dictate that they are capable of precision and stable operation, including on inclined surfaces. For example, a mobile platform vehicle must be capable of smooth transitions between stopping and starting, even on inclined surfaces, when supporting a worker on the extended platform. Accordingly, such vehicles will typically include a rechargeable electric power source, an electrically powered hydraulic fluid pump, a fluid tank, and one or more hydraulic motors coupled to the vehicle's drive wheels. Conventionally, hydraulic motors used on vehicles are combined with spring brakes that are normally engaged and are disengaged in response to a flow of hydraulic fluid that pressurizes the brake release mechanism.
Typically, hydraulic vehicles will employ a joystick to allow the user to control the starting, stopping, and directional movement of the vehicle. Accordingly, when the joystick is in the neutral position, the pump is off, the brakes are engaged, and the vehicle should remain in a stationary position. When the joystick is moved from its neutral position, the pump and motors are activated, and movement of the vehicle is generated in accordance with the position of the joystick.
All of the hydraulic components in such a system are interconnected in one or more hydraulic circuits that include valves, pressure sensors, and other components to control and direct the flow of hydraulic fluid as needed. A variety of different hydraulic circuits have been specifically designed to control hydraulic brakes and hydraulic motors used in industrial vehicles. The objective of such a circuit includes controlling the motors and brakes in a manner that provides precise, stable, and safe movement of the vehicle. Unfortunately, conventional hydraulic circuits have ranged from the effective but complicated to the ineffective. In particular, conventional hydraulic circuits have not been effective in counteracting and suppressing undesirable and unsafe movements of industrial vehicles inherently associated with hydraulic motors and brakes, and particularly associated use of the vehicle on an inclined surface.
For example, in most conventional systems, the hydraulic motor will be activated before the hydraulic brake is disengaged for the motor. In such case, the hydraulic motor begins to turn and drive into the hydraulic brake before the hydraulic brake is disengaged. This reduces the life of the brake and increases the wear and tear on the motor.
Additionally, most conventional systems fail to properly pressurize the hydraulic motor when the hydraulic motor is under a load. When these conventional systems experience a static load on the motor, the motor will freely turn and not support the load. Due to this fact, in these conventional systems a counterbalancing or counter braking measure must be used in order to maintain the load in a stationary position. Normally, the conventional systems are designed to engage the brake to the motor when the motor is operating at low speeds in order to support this load. Once again, this causes the motor to drive into the brake and reduces the useful life of the brake and the motor. Examples of instances where the support of the load is a problem in conventional systems include the suspension of a weight or the retardation of a vehicle on an incline. Attempts in the prior art to inhibit vehicle “runaway” on an inclined surface have not been satisfactory because of their complexity and/or inability to prevent chugging or surging movement of the vehicle as the motor and brake start and stop the vehicle in rapid succession.
Conventional hydraulic motors and related circuits also experience pressure variances that adversely affect the operation of the hydraulic motor and vehicle. Various types of pressure variations are present in most hydraulic motors and the conventional hydraulic systems that control those motors. For example, some of the pressure variances are commonly referred to as cogging, pulsing, or surging of the motor. These pressure variances greatly reduce the control of the motor, especially at low speeds, reduce the efficiency of the motor, and reduce the useful life of the motor. Of course, pulsing or surging of the motor can be unsafe for the operator if the cogging or pulsing translates to movement of the vehicle itself.
What is needed, then, is a hydraulic control circuit and method for efficiently and safely controlling a hydraulic brake and hydraulic motor. In particular, a hydraulic motor and brake control circuit is needed to effectively eliminate vehicle runaway, undesirable vehicle movements caused by pressure variations in the motor, and to event the motor from driving into the brake.